In an optical transmission system, specifically in an optical amplifier transmission system, amplified spontaneous emission (ASE) noises are accumulated due to the optical amplification in the course of optical transmission. The ASE noises within a band different from the signal wavelength band are easily removed by a band limited optical filter. However, this type of band limited optical filter is incapable of removing the ASE noises being superimposed on the signal wavelength band.
An ASE noise accumulated within a signal band deteriorates an optical signal to noise ratio, i.e. OSNR, of a signal light. FIG. 2 shows a spectrum example of a signal light in which an ASE noise is accumulated. The horizontal axis expresses the wavelength, and the vertical axis expresses the optical intensity. The solid line shows the optical intensity of the signal component, and the broken line shows the optical intensity of the noise component. However, the actual intensity of the signal component is obtained by subtracting the intensity of the noise component expressed by the broken line from the intensity of the signal light expressed by the solid line. The noise light within the signal band deteriorates a bit error rate, i.e. BER of the signal light.